The present invention concerns a process and an apparatus for printing at least once on flat and :possibly relatively inflexible individual articles whose surface to which printing is to be applied is substantially flat.
It will be noted here in this respect that the individual articles are primarily those whose extent perpendicularly to the plane of surface which is to be printed upon is substantially smaller than in the transverse direction thereto. Examples of such articles that may be mentioned are CDs, credit cards, telephone cards, control cards and like articles, in relation to which a high degree of safeguard against forgery is possibly also expected.
It is known for example for CDs to be decorated using a screen printing process. Such a process affords the advantage that the applied layer of printing is comparatively thick, being for example of the order of magnitude of 12.mu., so that when using conventional printing inks, good covering and print images are produced, which in a given respect satisfy high levels of requirement in terms of quality. The screen printing process also has the advantage when applying printing to CDs and other articles which have manufacturing tolerances in regard to their thickness that it compensates for such variations in thickness in the printing operation without involving additional steps as the printing stencil through which the printing ink, or another agent, is applied by means of a doctor to the article experiences a certain amount of elastic deformation due to the action of the doctor, and that deformation compensates for the inaccuracies caused by the variations in thickness, in regard to the position of the surface to which the printing is to be applied.
The requirements in regard to nature and quality of the print images on CDs and other individual articles are however becoming ever increasingly severe, in particular in regard to the possible configurations involved with the print image. That applies for example in regard to the reproduction of illustrations with a fairly high degree of detail fidelity, as well as shades of coloring in particular in the case of pictorial representations. The screen printing process generally does not adequately fulfill such requirements, especially as it only permits a relatively coarse screen printing pattern to be produced.
It might see that an offset printing process could be used for printing on individual articles as indicated above as such a process, for example by the use of a print configuration with a high degree of resolution, makes it possible to apply print images with a high level of detail fidelity in a large number of colors and intermediate tones, with the result that the number of colors and color shades in the finished print image can go far beyond the number of basic printing inks used for producing the print image. These are advantages and qualities of the offset printing process, with which any man skilled in the art is familiar and which therefore do not need to be described in greater detail here.
However the use of an offset printing process in relation to CDs and other flat articles which are subject to fluctuations in terms of their thickness hitherto fell foul of the fact that the offset printing process, to achieve good quality of print, requires accurate alignment of the printing or blanket cylinder and therewith the peripheral surface thereof, from which the print image is transferred on to the article to be printed upon, in relation to the article. That is in substance to be attributed to the fact that the application of ink in the offset printing process is substantially thinner than when using a screen printing process and for example is only of the order of magnitude of about 3.mu.. Particularly when using the above-mentioned pattern printing procedure which makes it possible to achieve the color shades referred to above, the crucial consideration, in terms of the quality of the resulting print image, is that transfer of the print image from the printing cylinder on to the article, in relation to the relative position of the printing cylinder and the article and therewith also in relation to the pressure which is operative between two members, satisfies the conditions which are the optimum conditions in that respect but which hitherto could not be satisfactorily achieved, having regard to the fluctuations in respect of thickness of the individual articles.
It may be noted at this point that an offset printing process is performed using a printing or blanket cylinder, on the cylindrical or peripheral surface of which is disposed a transfer means in the form of a printing blanket which is of a thickness of a few millimeters, for example 3 millimeters, and which generally comprises rubber material. Partial print images are produced by plate cylinders on the outer surface of the transfer means or blanket, that surface acting as a transfer surface. The partial print images are combined on the transfer surface to form an overall print image which is then transferred on to the article to be printed upon, in a single printing operation. The structure of an offset printing mechanism for carrying out that process and the way in which the offset printing process is performed are well known and therefore do not need to be described in further detail here. In regard to the above-indicated problems therefore, to achieve good print quality, the important consideration is essentially that the transfer surface of the transfer means which is generally in the form of a printing blanket is disposed during the printing operation in the optimum position relative to the surface of the article, which is to be printed upon, in order to provide that the print image is transferred from the transfer surface on to the article in a way which guarantees high quality in the finished print image.